Early tooth development, gestation, and season of birth in mammoths

The Yamal mammoth calf “Lyuba” and the Oimyakon mammoth calf are two recently collected, mummified woolly mammoth calves both dating to around 41 ka. They offer opportunities to study the earliest parts of mammoth life history through microstructural and compositional analyses of tooth dentin. Understanding mammoth paleobiology is critical for assessing how populations might have responded to ecological changes in the late Pleistocene, and such assessment will aid in determining the likely cause(s) of extinction. The state of tooth eruption and wear in Lyuba suggests that she died quite young. Prominent constrictions in the roots of the dP2s and dP³ correspond to neonatal lines in the dentin of each tooth, marking the time of birth. The neonatal line in dP² allowed us to estimate the duration of prenatal development, determine Lyuba’s age at death, and place birth in the context of variation in isotopic and elemental composition of the dentin. Growth increments and seasonal variation in δ¹⁵N indicate that Lyuba’s dP² records over a year of prenatal development, suggesting duration of gestation was similar to that observed in elephants. Death occurred at about 30–35 days, and birth appears to have occurred in spring. The Oimyakon specimen represents an individual older (biologically) than Lyuba. The dP2s have been lost, and most plates of the dP3s are in wear. An unerupted permanent tusk is present, but no deciduous tusk is present in the single recovered premaxilla. The tusk exhibits a circumferential ridge at mid-length that represents the surface expression of a prominent growth line in the dentin. Based on its prominence and elevated dentin Zn/Ca in its vicinity, this is interpreted as a neonatal line. Its occurrence in the tusk is of interest because this implies initiation of permanent tusk mineralization months before birth— yet permanent tusks are not present in Lyuba. Differences in timing of tusk development may result from individual, population, or sexual variation. Growth increments in the tusk indicate death at an age of about 7 months with both δ¹⁵N and δ¹³C generally declining postnatally. The increment-based age at death is less than an estimate based on the state of cheek tooth eruption and wear (using extant elephants as references), implying that ages based on comparisons to elephants may be inaccurate. Counts of dentin growth increments combined with identification of seasonally varying aspects of dentin composition could be used to calibrate the cheek tooth aging system for Mammuthus primigenius.